Fix infinite loop in del_post_hook (#21914)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21914
https://github.com/pytorch/pytorch/pull/21591 added a needed feature to clean up grad accumulator post hooks when the DistributedDataParallel model object is cleaned up. There's a minor typo that causes it to loop infinitely over the first element.
Differential Revision: D15878884
fbshipit-source-id: b7fd0bbd51eb187579d639b1709c6f7b62b85e7a
